Blowing of fluoropolymers, in particular of perfluoropolymers, is a well assessed technique used in particular for enhancing insulation properties in cable sheathing applications, while targeting light-weighting and substantial reduction of potentially flammable material.
Foaming of fluoropolymers is generally achieved through physical foaming, i.e. by introducing suitable gaseous components, generally referred to as blowing agents, in the molten mass of polymer, or by chemical foaming, i.e. submitting to heat treatment a composition comprising said fluoropolymers and a chemical reagent able to decompose yielding volatile decomposition products, so as to simultaneously melt the polymer and in situ generate the blowing agent.
In both these approaches, the use of a nucleating agent is considered as crucial for achieving homogeneous and regular formation of cells generated by the blowing agent. The use of nucleating agents generally results in a greater number of finer cells than would form without a nucleating agent.
Thus, U.S. Pat. No. 5,023,279 (DU PONT [US]) 11 Jun. 1991 discloses nucleating agents for foaming fluoropolymers selected from free acids or salts of partially or totally fluorinated aliphatic sulphonic or phosphonic acids, which optionally may contain, inter alia, ether oxygen, in particular complying with general formula: Z(CF2)x(CF2CFX)p(R′)y(CH2)zRO3]nM wherein Z is CCl3, CCl2H, H, F, Cl or Br;
each of X is selected from H, F, Cl and CF3;
R is S or P;
M is H or a metallic, ammonium, substituted ammonium or quaternary ammonium cation;
x is an integer between 0 and 20;
p is an integer between 0 and 6;
y is 0 or 1;
z is an integer between 0 and 20;
x+y+z+p is an integer or if zero, Z is CCl3 or CCl2H;
n is the valence of M; and
R′ is a C5-6 alicyclic ring diradical;
a C1-16 perfluorinated aliphatic polyether radical;
a substituted or unsubstituted aromatic diradical.
Among suitable compounds useful as nucleating agents, mention is made, in a long list of:                salts of telomer sulphonic acids of formula: F(CF2)n—CH2CH2SO3X, with n=6, 8, 10, 12 (which are preferred embodiments);        salts of perfluoroalkyl sulphonic acids;        Ba salt of perfluoro(2,5-dimethyl)-3,6-dioxamidecanoate: C7F15OCF(CF3) CF2OCF(CF3)COOX        Ba salt of perfluoro-3,5,7,9,11,13-hexaoxatetradecanoate: CF3OCF2OCF 2OCF2OCF2OCF2OCF2COOX.        
Similarly, U.S. Pat. No. 5,610,203 (DU PONT [US]) 11 Mar. 1997 discloses the use of compounds of formula Z(CF2)x(CF2CFX)p(R)y(CH2)zRO3]nM as nucleating agents in perfluoropolymers. All working embodiments are based on the use of Barium salts of mixture of perfluoroalkylethane sulfonic acids.
Thus, in the wide class of compounds described in above referred references, most of actual working embodiments are based either on fluorotelomer sulphonates, perfluoroalkyl sulphonates or on carboxylates.
While perfluoroalkylsuplhonates and fluorotelomer sulphonates (possibly degrading towards fluoroalkyl-containing derivatives) have raised or are currently raising environmental concerns, carboxylate derivatives have been found inappropriate for withstanding certain high temperature processing conditions, when used as nucleating agents.
There is thus a current shortfall in the art for the identification of alternative nucleating agents for fluoropolymers, which possess a better environmental profile, which are endowed with adequate thermal and chemical stability and which provide for outstanding behaviour in foaming, yielding high void fraction and thus low apparent densities of foamed material and regular cell distribution.
On the other side, fluorinated sulfonic acid derivatives comprising oxygen atoms in side chains are known in the art.
Also, US 2010120980 (DU PONT) 13 May 2010 discloses certain sulphonic compounds of formula:Rf—O—(CXX′)m—(CY2)n—SO3Mwith Rf C1-4 perfluoroalkyl;
X, X′ H or F, provided at least one of them being F;
Y being H or F;
m=1-4;
n=1-2
M=H, NH4, Li, Na or K.
Preferred and exemplified embodiments are those wherein Y=H and n=2 (i.e. telomers). These compounds are taught as suitable for several field of use, including, notably as foaming agents; no mention is made of their use as nucleating agents, in particular as nucleating agents for perfluoropolymers.
Similarly, EP 0466483 A (AUSIMONT SPA [IT]) 15 Jan. 1992 discloses a process for manufacturing perfluoroalkoxysulphonic compounds of formula: (Rf2CF2—SO2X)p. No mention is made in this document of the use of such compounds as nucleating agents for foaming perfluoropolymers.
Finally, JP 1065113 (ASHAI GLASS CO LTD) 10 Mar. 1989 discloses a molded product which is excellent in heat resistance and elongation characteristics, by using a specified diaminobenzene derivative as a chain extender in the reaction injection molding of a high-MW active hydrogen compound and a polyisocyanate. A starting material comprising said chain extender, a high-MW active hydrogen compound and optionally added another chain extender, catalyst, foaming agent, etc. and another starting material comprising a polyisocyanate compound (e.g., tolylene diisocyanate) are submitted to reaction injection molding to give a synthetic resin molding.